BAW resonators are suitable, in particular, for band-pass high-frequency filters in modern filter technology, and can be used, for example, in mobile communication devices.
A resonator operating with bulk acoustic waves has a piezoelectric layer that is disposed between two metal layers (electrodes). A sequence of layers can also be used instead of only one piezoelectric layer. The layers are deposited consecutively on a substrate and structured into resonators, that are electrically connected to one another and together can constitute, for example, a filter circuit especially a band-pass filter. Such a band-pass filter can also be used together with another filter in a duplexer.
FIG. 1 shows the equivalent circuit diagram of a BAW resonator. Outside a frequency range surrounding the resonant frequency, the resonator is characterized by a static capacitor C0 and, in proximity to the resonant frequency, by the series connection of a resistor Rm, a capacitor Cm and an inductive resistor Lm. The static capacitor is essentially defined by the resonator surface area and the thickness of the piezoelectric layer. The resistor Rm describes losses in the resonator, while the capacitor Cm and the inductive resistor Lm determine the resonant frequency
      f    r    =            1              2        ⁢                                  ⁢        π        ⁢                                            L              m                        ⁢                          C              m                                            .  
The ratio Cm/Co determines the coupling of the resonator. The coupling coefficient k of the resonator is linked to the resonant frequency fr and the antiresonant frequency fa:
            k      2        =                            f          a          2                -                  f          r          2                            f        a        2              ,wherein fo=fm√{square root over (1+Cm/Co)}.
A band-pass filter is characterized by a transfer function that has, in particular, a pass band and several stop bands. The pass band is, in turn, characterized by its bandwidth, the insertion attenuation in the pass band and the edge steepness at the edge of the pass band.
Two BAW resonators SR1 and SR2 (as depicted schematically in FIG. 2) can be acoustically coupled with one another if, for example, they are arranged in a stack one on top of the other. In this connection, the resonators form a series connection between a port P1 and a port P2, e.g., in a stacked-crystal arrangement, in which two resonators share a common electrode, that is connected to ground (see FIG. 3), or are arranged in a coupled-resonator arrangement, in which a coupling layer KS is arranged between the upper electrode E2 of the lower resonator and the lower electrode E3 of the upper resonator, and said electrodes are connected to ground (see FIG. 4). A first resonator in FIG. 3 comprises a piezoelectric layer PS1, that is arranged between two electrodes E1 and E2, and an acoustic mirror AS arranged below the electrode E1, said acoustic mirror resting on a carrier substrate TS. Above the first resonator, a second resonator is arranged that comprises a piezoelectric layer PS2, which is arranged between the electrode E2 and an electrode E3. Electrode E1 is connected to port P1, electrode E3 to port P2 and electrode E2 to ground.
The layer system shown in FIG. 4 includes a first resonator arranged on a carrier substrate TS, a coupling layer KS disposed above it and a second resonator arranged above the coupling layer KS. The first resonator is arranged as described in FIG. 3 and is connected between port P1 and ground. The second resonator contains (from bottom to top) two electrodes E3 and E4 and a piezoelectric layer PS2 arranged between said electrodes, the second resonator being connected between port P2 and ground. The coupling layer KS arranged between said resonators provides for acoustic coupling between these resonators.
Filters constructed of acoustically coupled resonators are characterized by a high stop band suppression. However, the edge steepness and, with it, the adjacent channel selectivity are comparatively low, due to the absence of defined pole positions in proximity to the pass band.
BAW resonators can be connected in a ladder-type or a lattice-type construction. The advantage of the lattice-type arrangement of the resonators in a band-pass filter is that the selection of such a filter in stop band areas well outside the pass band is very good, ranging, for example, between −40 and −60 dB. The disadvantage of this filter arrangement includes a low edge steepness of the pass band. For this reason, it may be difficult, in this type of filter arrangement, to achieve sufficient attenuation of the signal in the stop bands in proximity to the pass band.
Considerable edge steepness is required in some applications. In the case of duplexers that are suitable for the PCS telecommunications standard, for example, a decline in the transmission function from ca. −3 dB to significantly below −40 dB within a frequency range of only 20 MHz must be guaranteed. Previously known band-pass filters that are constructed of BAW resonators may not satisfy such requirements, due to additional frequency shifts in the edges in response to temperature change or as a result of existing production tolerances (which, in the case of a filter operating at ca. 2 GHz and having BAW resonators that contain a piezoelectric layer of ALN, can amount to several MHz).
It is known, from the reference EP 0949756 A2, that a series connection of stacked resonators acoustically coupled with one another, as well as additional resonators instead of only one resonator in a filter circuit, improves edge steepness in the transmission band of the filter. The disadvantage of this solution, however, is that it requires a great deal of space.